Field of the Invention
The present invention is directed to implantable medical devices, and more particularly to recharging of batteries in implantable medical devices.
Description of the Related Art
For electrical medical devices that are surgically implanted within the body of a patient, a recent improvement is the development of rechargeable batteries. Rather than undergoing surgery to remove or replace a device that has a dead battery, a patient may now undergo periodic recharging sessions, which are far less invasive and far less expensive.
For instance, a typical first-generation implantable hearing aid may last around six to nine years with its non-rechargeable battery. More recently developed hearing aids that use rechargeable batteries may last only about six to twelve weeks between recharging sessions, but the recharging sessions are simple, quick, inexpensive, non-invasive, and are completely painless. These rechargeable batteries may prolong the life of the device substantially, and the patient may be able to use the same implantable hearing aid for up to 20, 30, 40 years or more without subsequent surgery.
The use of rechargeable batteries in implantable devices required the development of a wireless power interface for recharging, using inductive coupling between two wire coils. Inductive designs have been used successfully in implantable medical devices such as defibrillators, insulin pumps, spinal cord stimulators, deep-brain stimulators and left-ventricular assist devices. Inductive designs have also been used successfully in commercial products such as cordless toothbrushes.
In an inductive design, current flowing in one coil causes a current to flow in an adjacent coil. For implantable medical devices, one coil (referred to as the “secondary” coil) is implanted under the skin of the patient, and the other coil (referred to as the “primary” coil) is in an external charger unit that is held against the skin or in close proximity of the patient during the recharging process.
The physical mechanism for energy transfer in an inductive design is similar to that used in transformers. A time-varying, typically sinusoidal, alternating current (AC) is passed through the primary coil. The time-varying current produces a time-varying magnetic field in the vicinity of the primary coil, which decreases in strength with increasing distance away from the primary coil. The time-varying magnetic field passes easily through the skin and tissue of the patient, and does not damage the skin or tissue in any way. The time-varying magnetic field causes a time-varying voltage to form in the secondary coil, and since the secondary coil is a good conductor, produces a time-varying current in the secondary coil. If the primary coil is driven at a particular frequency, such as 10 kHz, 100 kHz or 1 MHz, then the current created in the secondary coil also flows at the particular frequency, namely 10 kHz, 100 kHz or 1 MHz. The current in the secondary coil is then rectified, regulated, and directed toward recharging the battery.
A detailed example, with circuitry, is provided in the article by PENGFEI LI and RIZWAN BASHIRULLAH, “A Wireless Power Interface for Rechargeable Battery Operated Medical Implants”, IEEE Transactions on Circuits and Systems—II: Express Briefs, October 2007, pp. 912-916, Vol. 54, No. 10, which is incorporated by reference in its entirety herein.
For any wireless power interface that transfers power from one coil to another, there is a sensitivity to alignment between the coils. Typically, the coils transmit power most efficiently when they are in close proximity, both laterally and longitudinally. As the lateral and/or longitudinal separations increase, the efficiency drops, meaning that a smaller fraction of power emitted from the primary coil is received by the secondary coil.
The highest efficiency, or fraction of radiated power that is received by the secondary coil, occurs for circular coils of the same size that are directly longitudinally adjacent to each other. The efficiency drops if the sizes and/or shapes are mismatched, and if the coils are separated longitudinally and laterally. In general, the primary and secondary coils are made as large as practical, and are located as close to the surface of the skin as practical. During an actual charging session, once a charger unit is placed, it typically doesn't move much, and the efficiency tends to be relatively stable over the length of the session. In practice, efficiencies of 15% to 80% are common.
For implantable medical devices, there are standards for a variety of quantities, including electric field strength, magnetic field strength, temperature, and many others. In particular, the temperature standard dictates that the temperature difference between the device and the surrounding tissue must be less than two degrees Celsius. Such a temperature requirement has direct implications for the electrical performance of the device.
While the device is charging, it is using a certain amount of power. For example, if the device charges at a voltage of 4.2 V and a current of 150 mA, the power consumed for charging is the product of the voltage and current, namely, 0.63 W. If the amount of power received by the secondary coil exceeds 0.63 W, the excess power is converted to heat at the implant. If left unchecked, such excess power can lead to overheating of the implant, which can exceed the mandated temperature standard and may even damage the tissue of the patient, which would be unacceptable.
There are known ways to compensate for this excessive received power, with two such examples being described below.
For the first example, the current in the primary coil is set at the factory so that the power received by the second coil never exceeds a particular value, even when the first and second coils are perfectly aligned.
We consider a numerical example. We assume that 0.2 W is the maximum excess power that can be safely converted into heat, and that 0.63 W is the power that goes into recharging the battery. Using these numbers, 0.83 W is the maximum amount of power that can be generated safely in the second coil. If the current in the second coil exceeds 0.83 W at any point, and the 0.63 W value remains constant, then more than 0.2 W is converted into heat, and the device is out of specification.
For the case when the primary and secondary coils are perfectly aligned, which produces the maximum current in the secondary coil for a given current in the primary coil, the primary coil current is set to produce a secondary coil power output of 0.83 W. In other words, the primary coil current is set at the worst case for thermal issues, which is the best alignment between primary and secondary coils. When the alignment between the primary and secondary coils is less than optimal, the secondary coil current is less than 0.83 W, and less than 0.2 W is converted into heat, which is within the specification.
Although this first example ensures that the amount of power converted into heat is within an acceptable range, the trade-off is that the charging time may be unacceptably increased. For instance, assume that the charging time is two hours for well-aligned coils. If the coils are misaligned for some reason during the charging session, such as due to a lateral or longitudinal displacement of the charger unit, the charging time may be unacceptably increased, such as to six or eight hours.
While this scheme may ensure that the there is no overheating of the implanted device or the surrounding tissue, the trade-off of potentially excessive recharging times is a shortcoming.
For the second example, the implanted device includes a control loop that uses an RF telemetry link to talk back to the charger unit. Based on the data received over the RF telemetry link, the charger unit adjusts the current in the primary loop so that the power received by the secondary loop is set to a predetermined value, such as 0.63 W. The currents may be adjusted based on periodic communications over the RF telemetry link, such as several times a second. In this manner, if the charger unit is initially placed away from its optimal location, the RF telemetry link increases the primary coil current so that the secondary coil current is at its desired value. Or, if the charger unit is moved during recharging and the coils become misaligned, the primary coil current may be increased accordingly so that the amount of current in the secondary coil remains roughly unchanged.
Use of the RF telemetry link ensures that there is no overheating of the implanted device or the surrounding tissue, while keeping the charging times down to reasonable values. However, the RF telemetry link itself adds additional size, complexity and cost to both the implanted device and the external charger unit, which is undesirable.
Accordingly, there exists a need for a mechanism that restricts the power received by the secondary coil, thereby preventing overheating of the implanted device and surrounding tissue, without introducing the additional hardware and expense of an RF telemetry link.
For reference, we review common electrical quantities and their respective units. Voltage, V, is in volts. Current, I, is in amperes. Charge, Q, is in coulombs, or amp-sec. Power, P, is in watts, or volt-amp. Resistance, R, is in ohms, or volts/amp. Capacitance, C, is in farads, or amp-sec/volt. Inductance, L, is in henries (plural of henry), or volt-sec/amp.